The origin of the serotonergic (5-HT) innervation of the nucleus tractus solitarius (NTS) was studied by injecting fluorescent dyes or enzymes which are transported in a retrograde direction into the NTS. The dorsal raphe nucleus was also ablated with electrolytic lesions and the effects on 5-HT levels in the NTS and other brain areas were determined by HPLC. Finally, unilateral removal of the nodose ganglion was tested for its effects on NTS 5-HT levels. The results from these studies leave open the question of 5-HT innervation of the NTS since neither raphe lesions nor nodose ganglionectomy altered NTS 5-HT levels or its metabolism in any detectable fashion. Furthermore, fluorescent retrograde dyes could not be reliably traced from the NTS to any major 5-HT nuclei.